Multiple-Input Multiple-Output (MIMO) antenna systems use multiple antennas in a transmitter and multiple antennas in a receiver to increase communication throughput and/or range at the same bandwidth and power consumption when compared to a Single-Input Single-Output (SISO) antenna system. For instance, typically, multiple antennas increase range in a system. Also, for instance, when multiple sets of antennas are used, multiple simultaneous data streams can be sent and received in the system, thereby increasing data throughput. One use for MIMO systems is to send multiple streams of the same data to a receiver with multiple antennas. A processing unit in the receiver then uses the redundant data streams to construct a more reliable copy of the data. In this way, MIMO systems can also be used to increase reliability of the data transfer.
In send/receive systems, there are typically at least two ways to operate a communication system. One way is referred to as “Time Division Duplexing” (TDD), which uses a single channel for both forward link (from the transmitter) and reverse link (to the transmitter). Different time slots are used for forward and reverse link transmissions. Another way is Frequency Division Duplexing (FDD), which uses a channel for the forward link and a channel for the reverse link, each separated by a guard band.
Whether using TDD or FDD, MIMO systems can benefit from pre-processing, which may include determining antenna selection and power allocation over beam vectors prior to a forward or reverse link data burst. Pre-processing is often accomplished by measuring qualities of a given channel and adjusting the next transmission accordingly. In a TDD system, a transmitter can measure the qualities of its channel by examining the reverse-link bursts. This is because forward and reverse links use the same channel. However, in FDD systems, it is only the receiver that receives a burst over the forward link channel. Thus, measurements of the forward link channel qualities are best measured at the receiver rather than at the transmitter.
Further, due to the time offset between a first forward burst and a second forward burst (in a FDD system) and between a reverse burst and a forward burst (in a TDD system), channel qualities at the time of their measurement are outdated by the time of the next burst for which pre-processing is done. Basing pre-processing on outdated information can often cause degradation of communication. Accordingly, various systems use measured channel qualities and apply an algorithm thereto in order to predict channel qualities at the time of pre-processing. This helps to ensure more relevant channel information is used in pre-processing.
For example, it is possible for a TDD system to employ a technique whereby the transmitter measures channel qualities and applies pre-processing immediately before the next forward burst using channel qualities predicted specifically for the burst time. This is in contrast to FDD systems, wherein forward channel quality information is measured at a receiver, and wherein the receiver usually has no knowledge of the timing of future forward bursts. This introduces at least two problems: 1) how to do pre-processing for the forward burst when the channel information is not at the transmitter, and 2) how to predict channel information when the timing of the next burst is not known.